The simultaneous actuation of the pairs of valves associated with a piston in an internal combustion engine is usually achieved by valve actuation mechanism that includes a valve bridge. The valve bridge is actuated by a rocker arm or like structure to contact simultaneously the terminal ends of the stems of the pair of valves and to cause the valves to reciprocate between open and closed positions. As long as both valves in the pair reciprocate freely, the valve actuation mechanism will continue to contact both valve stems simultaneously. Both valves will continue to open and close as required for normal engine operation, and the loads placed on the valve bridge and valve stems by this process will be substantially evenly distributed. If, however, something occurs to interfere with the simultaneous movement of both valves by the valve bridge, distribution of the load on the valve bridge and associated structures is unequal. This can lead to the application of forces and stresses on these structures which could ultimately cause major damage or possibly engine failure.
The simultaneous contact of the valve stems by the valve bridge would not occur if, for example, one of the valves became stuck in a closed position or otherwise rendered immovable during engine operation. In this event, the height of the immovable valve stem would be fixed above the engine block while the height of the movable valve stem would vary. The valve bridge would then forcibly contact the immovable valve stem, but might or might not also contact the movable valve stem. This forcible contact between the valve bridge and valve stem could force the valve bridge out of alignment and transfer undesirable forces to the rocker arm and associated structures. As a result, the prior art has proposed structure to maintain the alignment of the valve bridge and associated components in an attempt to avoid these undesirable loads.
In certain valve actuating mechanisms, the valve bridge is guided intermediate its ends for reciprocating movement to operate a pair of valves in the manner shown, for example, in U.S. Pat. No. 3,963,004, issued June 15. 1976 to Glenn W. Lysinger and James S. Sears and in U.S. Pat. No. 3,633,556, issued Jan. 11, 1972 to Atusushi Inoue. In such guided valve actuating mechanisms, the valve bridge typically includes a machine guide or slider from which extends a shank portion which must slidably engage a guide rod or guide pin secured into the cylinder head of an engine. In addition, separate guide sleeves are also provided for each valve to maintain the axial alignment of the valve during engine operation. Such features of construction, operation and arrangement can render the valve actuating mechanism difficult and expensive to manufacture, and can undesirably add weight to the engine. As a result, guideless valve bridges which eliminate this structure have been proposed.
A valve actuating mechanism of the type in which a guideless valve bridge is used to control operation of a pair of valves in an internal combustion engine is shown in U.S. Pat. No. 4,327,677. issued May 4, 1982 to Arnold Vander Bok. Vander Bok discloses a valve actuating mechanism in which a guideless valve bridge and a pair of valves are provided with cooperating means whereby alignment of the valve bridge relative to the valves is maintained and the valve bridge is permitted to automatically compensate for any variation in valve stem height. This valve actuating mechanism eliminates some of the problems associated with previously available mechanism. However, a serious concern remains with this type of guideless valve bridge. In the event that one of the two valves sticks to its associated valve seat in the closed position, the remaining non-sticking valve in the pair will complete an opening stroke which is nearly twice the distance of a normal opening stroke at twice the typical rate of acceleration. This can lead to collisions between the valve and the piston with the potential for major and possibly catastrophic internal engine damage. None of the known valve actuating mechanisms includes structure that will prevent such an occurrence.
Thus, there is a need for a valve actuating mechanism of the type including a guideless valve bridge for a valve stop structure which limits the maximum travel during the opening stroke of each of the valves, thereby preventing collisions between the valves and the pistons and reducing the potential for internal engine damage.